The present invention relates to a lightweight, acidproof, hydrolysis-resistant optical and ophthalmic glass having an index of refraction n.sub.d .gtoreq.1.69, an Abbe number v.sub.d .gtoreq.37, and a density s.ltoreq.3.30 g/cm.sup.3. It is a borosilicate glass in the system SiO.sub.2 -B.sub.2 O.sub.3 -CaO-TiO.sub.2.
Lightweight glasses of a high refractive index, for example for ophthalmic optics or for optical systems have been described in DAS [German Published Application] No. 2,259,183. These glasses, having a density of about 3.0 g/cm.sup.3, are especially well suited for strong corrections, but exhibit the grave disadvantage of strong dispersion, i.e., they have a comparatively low Abbe number .ltoreq.33. This strong dispersion causes a person wearing glasses of this type to see pronounced color fringes. For this reason, attempts have been made to improve their dispersion while retaining the same or only a slightly increased specific density (French Pat. No. 2,427,309). These new glasses, however, also fail to meet the requirements for a v.sub.d value .gtoreq.37 which generally would be highly desirable.
In further developments, glasses were provided having an increased Abbe number; however, they cannot be manufactured continuously due to their instability toward crystallization. In addition, their chemical resistance (acidproofness and resistance to climatic changes) do not satisfy the requirements imposed on spectacle lenses. Glasses of this type are described in DOS [German Unexamined Laid-Open Application] No. 2,809,409 and DAS No. 2,715,894, as well as DAS No. 2,623,683.
Accordingly, it is an object of the present invention to provide a new optical and ophthalmic glass which meets the aforementioned requirements and avoids the aforementioned disadvantages.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
These objects have been attained by providing an optical and ophthalmic glass having an index of refraction of .gtoreq.1.69, (e.g., 1.69-1.71), an Abbe number of .gtoreq.37, (e.g., 37-43) and a density of .ltoreq.3.30 g/cm.sup.3, (e.g., 3.30-3.10), comprising (in % by weight):
______________________________________ Preferred Range, General Range When Included ______________________________________ SiO.sub.2 18-36 18-36 B.sub.2 O.sub.3 3-18 3-18 GeO.sub.2 0-8 0-8 Al.sub.2 O.sub.3 0-6, 0-6 wherein SiO.sub.2 + B.sub.2 O.sub.3 + Al.sub.2 O.sub.3 + GeO.sub.2 = 25-39 P.sub.2 O.sub.5 0-14, 0.5-4 wherein SiO.sub.2 + B.sub.2 O.sub.3 + Al.sub.2 O.sub.3 + GeO.sub.2 + P.sub.2 O.sub.5 = 30-44 Li.sub.2 O 0-12 0-12 Na.sub.2 O 0-14 0-14 K.sub.2 O 0-18 0-18 Li.sub.2 O + Na.sub.2 O + K.sub.2 O = 2-18 2-18 MgO 0-15 0-15 CaO 7-25 7-25 MgO + CaO 15-32 15-32 SrO 0-13 0-13 BaO 0- 17 0-17 ZnO 0-15 0-15 PbO 0-10 0-10 SrO + BaO + ZnO + PbO 0-20 TiO.sub.2 3-14 3-14 Nb.sub.2 O.sub.5 0-13 0-13 WO.sub.3 0-10 0-10 TiO.sub.2 + Nb.sub.2 O.sub.5 + WO.sub.3 = 10-21 ZrO.sub.2 0-15, 3-8 La.sub.2 O.sub.3 0-20 3-12 Y.sub.2 O.sub.3 0-16 0-16 Gd.sub.2 O.sub.3 0-16 0-16 Yb.sub.2 O.sub.3 0-10 0-10 La.sub.2 O.sub.3 + Y.sub.2 O.sub.3 + Gd.sub.2 O.sub.3 + Yb.sub.2 O.sub.3 = 0-20 3-12 Ta.sub.2 O.sub.5 0-13 0-13 HfO.sub.2 0-5 0-5 SnO.sub.2 0-6. 0-6 ______________________________________
A particularly preferred such glass has a v.sub.d value of .gtoreq.39 and a density s of .ltoreq.3.2 g/cm.sup.3, is sufficiently stable with respect to crystallization to permit continuous manufacture and simultaneously satisfies the minimum requirements for acid resistance and resistance against climatic changes, and comprises (% by weight):
______________________________________ B.sub.2 O.sub.3 5-14 SiO.sub.2 24-32 Al.sub.2 O.sub.3 0-6 GeO.sub.2 0-8 B.sub.2 O.sub.3 + SiO.sub.2 + Al.sub.2 O.sub.3 + GeO.sub.2 = 33-39 P.sub.2 O.sub.5 0-6 B.sub.2 O.sub.3 + SiO.sub.2 + Al.sub.2 O.sub.3 + GeO.sub.2 + P.sub.2 O.sub.5 = 37-43 Li.sub.2 O 0-7 Na.sub.2 O 0-8 K.sub.2 O 0-11 Li.sub.2 O + Na.sub.2 O + K.sub.2 O = 4-14 MgO 0-10 CaO 12-25 MgO + CaO = 18-27 ZnO 0-8 BaO 0-6 SrO 0-6 PbO 0-5 ZnO + BaO + SrO + PbO = 0-8 TiO.sub.2 6-10 Nb.sub.2 O.sub.5 0-8 WO.sub.3 0-10 TiO.sub.2 + Nb.sub.2 O.sub.5 + WO.sub.3 = 13-19 ZrO.sub.2 3-9 La.sub.2 O.sub.3 0-11 Y.sub.2 O.sub.3 0-12 Gd.sub.2 O.sub.3 0-8 Yb.sub.2 O.sub.3 0-7 La.sub.2 O.sub.3 + Y.sub.2 O.sub.3 + Gd.sub.2 O.sub.3 + Yb.sub.2 O.sub.3 = 0-14 Ta.sub.2 O.sub.5 0-7 HfO.sub.2 0-6 SnO.sub.2 0-6 ______________________________________